These “off-target” genetic alterations demonstrate that certain CRISPR base editors need further refinement in a research finding of interest to pathologists

Could CRISPR
DNA-editing technology unintentionally effect RNA as well? A new study conducted
at Massachusetts General Hospital
(MGH) suggests that it can. Clinical
laboratories doing genetic testing will want to understand why this
research implies that refinements to CRISPR may be needed for it to be accurate
in therapeutic applications.

For years, a huge value of CRISPR (Clustered Regularly
Interspaced Short Palindromic Repeats) base editors have been their ability to
edit genes or convert a specific DNA base without breaking the DNA. Now, the MGH
scientists have discovered that certain CRISPR base editors may extend beyond
the targeted DNA and perform unwanted edits to RNA, according to a news release.

“Most investigation of off-target base editing has focused
on DNA, but we have found that this technology can induce large numbers of RNA
alterations as well. This surprising finding suggests the need to look at more
than just genetic alterations when considering unintended off-target effects of
base editors in cells,” J. Keith Joung, MD,
PhD, MGH Pathologist and Professor of Pathology at Harvard Medical School, stated in the news release.

The researchers had set their sights on developing a base
editor that targets cytosine,
according to the study.

“Previous studies of cytosine base editor specifically have
identified off-target DNA edits in human cells. Here, we show that a cytosine
base editor with rat APOBEC1
[rAPOBEC1] enzyme can cause extensive transcriptome-wide RNA
cytosine deamination in
human cells,” the scientists wrote in Nature.

According to the news
release, when the researchers put base editors into human liver and kidney cells,
they found their technology induced efficient edits at the target DNA site.
However, they also discovered tens of thousands of cytosine-to-uracil edits in the cells. They
found that deaminases, an enzyme that acts as a catalyst, which they used in
their base editor to change DNA, also altered the RNA in the cells, Science reported.

“Base editors are still incredibly powerful tools. This is just another parameter we need to understand,” J. Keith Joung, MD, PhD (above), MGH Pathologist and Professor of Pathology at Harvard Medical School, told Science. (Photo copyright: Massachusetts General Hospital.)

The researchers developed a way to reduce the unwanted RNA
edits, while maintaining the targeted DNA effects. They came up with cytosine
base editor variants, which they dubbed SElective Curbing of Unwanted RNA
Editing (SECURE).

“We engineered two cytosine base editor variants bearing
rAPOBEC1 mutations that substantially decreased the number of RNA edits in
human cells,” the researchers wrote in their study.

However, they also
called for changes to how base editors are used. “For research applications,
scientists using base editors will need to account for potential RNA off-target
effects in their experiments,” the MGH news release notes. “For therapeutic
applications, our results further argue for limiting the duration of base-editor
expression to the shortest length of time possible and the importance of
minimizing and accounting for potential impacts of these effects in safety
assessments.”

Other Studies Explore CRISPR

Other studies published earlier this year on mice and on rice also suggested
that “modified CRISPR-Cas9 technology will need to be further refined before it
can safely be used for research and therapeutic applications,” The Scientist reported.

Clinical laboratory leaders and pathologists recognize
CRISPR technology is changing the way research is done for diagnosing disease
as well as guiding treatment. Dark Daily has reported on key
CRISPR developments over many years.

And now, though the MGH study may appear to be a set-back
for CRISPR, it also may propel further research into possible therapeutic
applications of CRISPR base editing. It’s a development worth watching.